1. Field of the Invention
This invention relates to a method for producing water absorbent resins, and more specifically, to a polymerization method for producing water absorbent resins having excellent absorption ability.
2. Description of the Prior Art
In general, the absorption ability of a water absorbent resin comprising a polymer of a water soluble monomer having a polymerizable unsaturated group as the main chain increases as the molecular weight of the main chain becomes larger. In order to provide a water absorbent resin having a high molecular weight by polymerizing a water soluble monomer and a crosslinking agent with a small amount of a polymerization initiator to an appropriate concentration, a rise in the temperature of the polymerizable monomer solution resulting from the polymerization heat should be avoided.
To realize a high molecular weight by avoiding the rise in temperature, methods of polymerizing while eliminating heat using a special polymerizing device to facilitate cooling off of the hydrogel obtained by the polymerization have been proposed, such as a method of polymerizing in a sheet form or a method of facilitating the cooling efficiency by the use of a device to stir for increasing the surface area to be cooled while pulverizing the proposed in JP-B-2-14361 and JP-A-4-175319.
However, in the above mentioned conventional production methods, heat elimination is not efficient enough to provide a water absorbent resin having a sufficiently high molecular weight. Thus, there have been problems including difficulty in producing a water absorbent resin having a good absorption ability and the need of a complicated production device.